Detailed introduction to the usage of PHP constants and variables_PHP tutorial

In the past, I rarely introduced the usage and reference table of variables, constants and magic constants in PHP in such detail. This article is of great help to beginners and friends who need to know more can refer to it.

Variables:

Variables are used to store values, such as numbers, text strings, or arrays.

Once a variable is set, we can use it repeatedly in the script.

All variables in PHP start with the $ symbol.

The correct way to set variables in PHP is:

代码如下	复制代码
$var_name = value;

Beginners to PHP often forget the $ sign in front of variables. If you do that, the variable will be invalid.

Let’s try to create a variable that holds a string, and a variable that holds a numeric value:

代码如下	复制代码
$txt = "Hello World!"; $number = 16; ?>

1. How to define variables, and how is it different from languages ​​such as C#?
Variables in PHP are represented by a dollar sign followed by the variable name. Variable names are case-sensitive. For example:

The code is as follows	Copy code
代码如下	复制代码
$var='Jim';   $VAR='Kimi;   echo "$var,$VAR";//输出“Jim,Kimi"

$var='Jim';

$VAR='Kimi;
echo "$var,$VAR";//Output "Jim,Kimi"

?>You may also care about the naming of variables, which is actually the same as in most languages.

2. Are variables case-sensitive?

代码如下	复制代码
1 2 $foo = 'Bob';              // 赋值'Bob'给foo 3 $bar = &$foo;              // 通过$bar引用.注意&符号 4 $bar = "My name is $bar";  // 修改 $bar 5 echo $bar; 6 echo $foo;                // $foo 也修改了. 7 ?>

As mentioned in 1, it is case sensitive.

Note, one thing that needs to be explained is that since PHP4, the concept of reference assignment has been introduced, which is actually similar to references in most languages, but I think the most similar one is C/C++ because it also uses the "&" symbol. For example:

The code is as follows

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1 2 $foo = 'Bob'; // Assign 'Bob' to foo
3 $bar = &$foo; // Referenced through $bar. Note the & symbol 4 $bar = "My name is $bar"; // Modify $bar

5 echo $bar;

6 echo $foo; // $foo has also been modified.

代码如下	复制代码
$a = "b";  $$a = "123";  echo $b; ?>

7 ?>

Like other languages, only variables with variable names can be referenced. Okay, now everyone should have a general understanding of variables. Now let's look at indirect references and string concatenation of variables. ①Indirect reference of variables: Let’s look at an example first

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$a = "b"; $$a = "123"; echo $b; ?>

The above output is 123

We can see that there is an extra $ in the second line of code, and the variable is accessed through the specified name. The specified name is stored in $a("b"), and the value of this variable $b is changed to 123. Therefore, such a variable of $b is created and assigned.

You can increase the number of references arbitrarily by adding an additional $ mark in front of the variable.

②String connection: Let’s look at an example first

代码如下	复制代码
$a = "PHP 4" ; $b = "功能强大" ; echo $a.$b; ?>

It should be noted that in PHP 4.2.0 and subsequent versions, the default value of the PHP instruction register_globals is off. This is a major change to PHP. Setting register_globals to off affects the global availability of the predefined set of variables. For example, to get the value of DOCUMENT_ROOT, you would have to use $_SERVER['DOCUMENT_ROOT'] instead of $DOCUMENT_ROOT. Another example, use $_GET['id'] instead of $id from the URL http://www.example.com/test Get the id value in .php?id=3, or use $_ENV['HOME'] instead of $HOME to get the value of the environment variable HOME

We see the third line of code, the English (period) sign, which can concatenate strings into a merged new string.

超全局变量	描述
$GLOBALS	包含一个引用指向每个当前脚本的全局范围内有效的变量。该数组的键名为全局变量的名称。从 PHP 3 开始存在 $GLOBALS 数组。
$_SERVER	变量由 web 服务器设定或者直接与当前脚本的执行环境相关联。类似于旧数组 $HTTP_SERVER_VARS 数组（依然有效，但反对使用）。
$_GET	经由 URL 请求提交至脚本的变量。类似于旧数组 $HTTP_GET_VARS 数组（依然有效，但反对使用）。
$_POST	经由 HTTP POST 方法提交至脚本的变量。类似于旧数组 $HTTP_POST_VARS 数组（依然有效，但反对使用）。
$_COOKIE	经由 HTTP Cookies 方法提交至脚本的变量。类似于旧数组 $HTTP_COOKIE_VARS 数组（依然有效，但反对使用）。
$_FILES	经由 HTTP POST 文件上传而提交至脚本的变量。类似于旧数组 $HTTP_POST_FILES 数组（依然有效，但反对使用）
$_ENV	执行环境提交至脚本的变量。类似于旧数组 $HTTP_ENV_VARS 数组（依然有效，但反对使用）。
$_REQUEST	经由 GET，POST 和 COOKIE 机制提交至脚本的变量，因此该数组并不值得信任。所有包含在该数组中的变量的存在与否以及变量的顺序均按照 php.ini 中的 variables_order 配置指示来定义。此数组在 PHP 4.1.0 之前没有直接对应的版本。参见 import_request_variables()。
$_SESSION	当前注册给脚本会话的变量。类似于旧数组 $HTTP_SESSION_VARS 数组（依然有效，但反对使用)

Constant:

A constant is an identifier (name) of a simple value. As the name implies, the value cannot change during script execution (except for so-called magic constants, which are not constants). Constants are case-sensitive by default. Normally constant identifiers are always uppercase.

Constant names follow the same naming rules as any other PHP tag. Legal constant names begin with a letter or an underscore, followed by any letters, numbers, or underscores. The regular expression is expressed as follows: [a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*

① is data that cannot be changed during program execution. The scope of constants is global.

②The naming of constants is similar to variables, but without the dollar sign "$". A valid constant name begins with a letter or an underscore, followed by any number of letters, numbers, or underscores.

③ Generally, constants in PHP are in uppercase letters and are divided into system constants and custom constants.

We have just briefly talked about system constants, which will be introduced later.

1. __FILE__ Default constant refers to the PHP program file name and path;
2. __LINE__ Default constant refers to the number of lines of the PHP program;
3. __CLASS__ The name of the class;

Custom constant: define a constant through the define() function,

The syntax format is: bool define ( string $name, mixed $value [, bool case_$insensitive] )

name: Specify the name of the constant.
value: Specifies the value of the constant.
insensitive: Specifies whether the constant name is case-sensitive. If set to true, it is case-insensitive; if set to false, it is case-sensitive. If this parameter is not set, the default value is false.

// Legal constant name
define("FOO", "something");
define("FOO2", "something else");
define("FOO_BAR", "something more");

//Illegal constant name
define("2FOO", "something");

// The following definition is legal, but should be avoided: (custom constants do not start with __)
// Maybe one day in the future PHP will define a __FOO__ magic constant
// This will conflict with your code
define("__FOO__", "something");

?>

几个 PHP 的“魔术常量”

名称	说明
__LINE__	文件中的当前行号。
__FILE__	文件的完整路径和文件名。如果用在被包含文件中，则返回被包含的文件名。自 PHP 4.0.2 起，__FILE__ 总是包含一个绝对路径（如果是符号连接，则是解析后的绝对路径），而在此之前的版本有时会包含一个相对路径。
__DIR__	文件所在的目录。如果用在被包括文件中，则返回被包括的文件所在的目录。它等价于 dirname(__FILE__)。除非是根目录，否则目录中名不包括末尾的斜杠。（PHP 5.3.0中新增） =
__FUNCTION__	函数名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该函数被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。
__CLASS__	类的名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该类被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。
__METHOD__	类的方法名（PHP 5.0.0 新加）。返回该方法被定义时的名字（区分大小写）。
__NAMESPACE__	当前命名空间的名称（大小写敏感）。这个常量是在编译时定义的（PHP 5.3.0 新增）

http://www.bkjia.com/PHPjc/629213.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/629213.htmlTechArticleIn the past, I rarely introduced the usage and reference table of variables, constants and magic constants in PHP in such detail. This article is of great help to beginners and friends who need to know more can...